The present invention relates to an ingot mold for continuous casting of molten metal, particularly for forming rectangular- or square-section steel billets.
Billets or ingots may be formed from molten metal, particularly steel, by continuous casting of the molten metal in a conduit-shaped mold (ingot mold) followed immediately by rolling (so-called xe2x80x9ccasting and direct rollingxe2x80x9d technique). While enabling fairly high output speeds and good-quality products, this technique currently poses several drawbacksxe2x80x94mainly due to the process whereby the molten material is cooled and solidified in the moldxe2x80x94which limit the maximum output speed obtainable and the range of products that may be produced from the same mold.
That is, excessively fast casting in a continuous ingot moldxe2x80x94through which the molten material is cooled and starts solidifyingxe2x80x94is known to result in the formation of defects in the finished product. More specifically, cooling, which occurs by the molten material yielding heat to the walls of the mold, is only fully effective if the material remains in contact with the walls, so that a surface layer of solid metal (so-called xe2x80x9cskinxe2x80x9d) is formed and gradually increases as the material travels through the mold. As it solidifies, however, the material contracts and is detached from the walls, particularly the inner edges, of the mold, so that air pockets are formed between the metal skin and the mold wall, thus reducing the amount of heat given off, and slowing down the solidification process. The metal skin (solid layer) therefore continues growing, except at the corners, where insufficient heat is subtracted, thus resulting in only partial solidification of the molten metal and in the formation of defects in the finished product.
This also increases the likelihood of cracks forming at the edges, due to undesired stress on the metal skin as it withdraws from the wall of the mold.
In short, the resulting billet is of poor torsional strength, is highly susceptible to the formation of longitudinal cracks along the edges, and has a very uneven skin temperature, all of which create problems at the subsequent rolling stage and invariably impair the quality of the finished product.
To overcome these drawbacks, a relatively low casting speed is maintained to enable the material to cool as uniformly as possible. Also, reducing the radius of curvature of the inside corners of the mold is known to reduce the formation of air pockets, again providing casting speed is not too high.
When casting only one type of material, detachment of the metal skin from the mold walls may be reduced by improving the geometry of the mold, e.g. using appropriately tapered molds. This solution, however, cannot be applied to molds for producing materials of different characteristics (e.g. different types of steel).
Finally, some known molds have a particular, e.g. concave-walled, section for the passage of the molten metal. While reducing the problem of detachment, however, this type of mold also fails to provide for increasing casting speed over and above certain limits.
It is an object of the present invention to provide an ingot mold for continuous casting of metal materials, designed to overcome the aforementioned drawbacks typically associated with known molds. In particular, it is an object of the invention to provide an ingot mold enabling effective cooling of the molten metal at all points and along the whole length of the mold, to prevent detachment of the metal from the walls, even from the edges, of the mold, and to enable high-speed casting of top-quality products allowing of immediate rolling.
According to the present invention, there is provided an ingot mold for continuous casting of molten metal, in particular for forming rectangular- or square-section steel billets, comprising a longitudinally elongated casting conduit through which said molten metal flows and is cooled; said casting conduit having a substantially rectangular cross section, and being defined by a first and a second pair of substantially parallel, side by side walls; said first and said second pair of walls being substantially perpendicular to one another, and defining four inner edges; characterized in that said casting conduit comprises at least a first portion having rounded inner edges and an inner section varying gradually from a respective longitudinal first end, at which said molten metal is fed in, towards a respective longitudinal second end opposite the first; said variation in section of said at least a first portion being determined by a corresponding continuous, gradual variation in the radius of curvature of said rounded inner edges; said radius of curvature decreasing from a maximum value at said first end, to a minimum value at said second end.
More specifically, said variation in section of said at least a first portion of said casting conduit is such as to reproduce a thermal contraction of said molten metal as it travels through said at least a first portion.
The particular design of the ingot mold according to the present invention provides for effectively cooling the molten metal at all the surface points, even at the inner edges, and along the whole length of the mold. The material, in fact, is maintained contacting the walls, even at the edges, of the mold at all times, by virtue of the particular inner section of the variable-section portion, which in point of fact reproduces the cooling pattern of the material as it cools and solidifies.
The metal is thus prevented from detaching from the walls, even from the edges, of the mold, with no need to maintain an excessively low casting speed. Cooling inside the mold according to the invention is so effective as to enable immediate direct rolling of the product issuing from the conduit, with no need for any intermediate processing, by virtue of the material having the required characteristics (in particular, no internal stress at the edges, and an even skin temperature). Moreover, the same mold may be used for different materials, e.g. for producing different types of steel, while at the same time ensuring high-quality products and maintaining a high output speed.